The present invention relates to a technique for adjusting an oscillation frequency of a voltage-controlled oscillator (VCO), and to, for example, a technique effective for application to a communication semiconductor integrated circuit employed in a transmission system of a wireless communication system, wherein a voltage-controlled oscillator is controlled based on transmit data while generating a carrier frequency signal (carrier) by the voltage-controlled oscillator to thereby frequency-modulate the carrier.
In the standard called “Bluetooth” that sets the rules related to the transmission of data between peripheral devices such as a personal computer, a printer, etc. by wireless communication, a frequency modulation system has heretofore been adopted which modulates a carrier frequency signal lying in a 2.4 GHz band with ±160 kHz and transmits data.
When such frequency modulation is performed, there is considered a system wherein a voltage-controlled oscillator for generating a carrier is controlled based on transmit data to modulate the frequency. As the voltage-controlled oscillator used in the modulation, there are known a circuit of such a type that the current is varied by a control voltage to thereby control an oscillation frequency, and an LC resonant voltage-controlled oscillator wherein the capacitance value of a variable capacitor is varied by a control voltage to thereby control an oscillation frequency.
The above voltage-controlled oscillator used in the wireless communication must be oscillated in a desired frequency range. However, the oscillation frequency of the voltage-controlled oscillator is often shifted from the desired frequency range depending on variations in manufacture. Therefore, it has heretofore been practiced to measure the frequencies of the voltage-controlled oscillator one by one by probe inspection at the final stage of a manufacturing process and adjust the capacitance values of capacitive elements constituting the voltage-controlled oscillator by trimming using laser or the like to thereby match up the frequencies. However, a problem arises in that an adjustment made to the frequencies by trimming in the manufacturing process will increase the manufacturing cost.
Since the capacitive elements formed on a semiconductor chip might vary in capacitance value with a change in the distance between electrodes due to stress applied to a chip, the oscillation frequency of the voltage-controlled oscillator might vary according to the form of packaging of the chip. There is also a fear that the oscillation frequency of the voltage-controlled oscillator will be shifted due to a time variation in device characteristic and a variation in temperature. Thus, the adjustment to the frequencies by trimming in the manufacturing process cannot provide the correction of the oscillation frequency of the voltage-controlled oscillator, which varies according to the form of packaging and is shifted due to a variation in operating environment.
Meanwhile, as the invention related to a PLL circuit capable of automatically adjusting a self-running frequency of the voltage-controlled oscillator, there has been proposed one provided with a comparator which compares a control voltage supplied from a loop filter to the voltage-controlled oscillator and a voltage used as the reference, and a circuit which generates trimming data, based on the result of comparison by the comparator (see, for example, a patent document 1).
Patent Document 1
Japanese Unexamined Patent Publication No. Hei 7(1995)-46123